The present invention relates to a vehicle navigation system, and, more particularly, to a system for computing an optimal route from a starting point to a destination and for guiding a vehicle according to the computed optimal route.
Generally, a vehicle navigation system computes the optimal route to a destination before the vehicle starts its journey. En route, the system computes local directions along the optimal route with reference to the vehicle's present location in real time. The system guides the vehicle's navigation from a starting point to a destination by displaying route information in real time on a CRT.
When a vehicle travels, it sometimes deviates from the optimal route. For example, assume a vehicle is traveling on a road in a particular direction. A route guidance display instructs the driver to turn right where the road he is on intersects a second road at right angles. However, the driver cannot make the turn because the second road is under construction. In such a case, the driver is forced to travel straight along the road he is on, without turning to the right at the next intersection. Thus the vehicle has deviated from the optimal route.
Under such circumstances, conventional navigation systems take the vehicle's present location as the starting point and recompute a new optimal route from the new starting point to the destination. A conventional navigation system thereafter guides local navigation along the recomputed optimal route.
Computation of an optimal route generally requires several seconds. Therefore, when a vehicle keeps on traveling after it has deviated, it moves forward by distance L from the new starting point during the time the new optimal route is computed, where L is given by the equation EQU L=V.times.T,
where V=the velocity of the vehicle and T=the time to compute the new optimal route.
Should the recomputed optimal route require a right turn at a second intersection where the road currently traveled intersects a third road, the vehicle has passed through the second intersection if the distance between the first and second intersections is shorter than the distance L. As a result, the vehicle is once more off the optimal route, which must be again recomputed. The difficulties caused by having to recompute successive optimal routes create drawbacks for the conventional navigation system.